This invention relates to a sliding current collector and more particularly a sliding current collector of the type suitable for a slip ring or a commutator of a rotary electric machine.
Generally, in electric machines utilizing electric energy, a sliding current collector is used for supplying a current to a moving part thereof, for example, for supplying a field current in a rotary-field type AC generator, supplying an armature current in a rotary-armature type DC motor and supplying electric power in an electric car.
The sliding current collector has a pair of current collecting members which are slidable relative to each other and electrically connected together for supplying a current from one to the other, and hence the condition in contact between sliding surfaces of the members is very important for providing good function and reliable operation of the sliding current collector.
Since it is unavoidable that the sliding current collector will be subject to wear when used for a long time, it is particularly designed in consideration of ease of maintenance and replacement. For example, one of the current collecting members which can be repaired or replaced only through time-consuming labor is made of a metallic material such as copper, steel or iron which is durable against wear while the other current collecting member is made of a material such as sintered copper powder which wears more easily than the one member. However, in the event that a spark is generated across the sliding surfaces owing to the electrical polarity (positive or negative) difference and defective sliding contact, these members undergo burn-out damage which grows with time or unforeseen abnormal wear occurs.
Under the circumstances, the present inventors have studied conductive ceramics which are durable against oxidization as a material for the paired sliding members of the current collector.
To prepare the conductive ceramics, a ceramic substrate, such as SiC (silicon carbide) or Si.sub.3 N.sub.4 (silicon nitride), is mixed with a conductive additive such as ZrB.sub.2 (zirconium boride), TiN (titanium nitride) and HfB.sub.2 (hafnium boride) at various ratios and the mixture is sintered at a high temperature. For example, when a mixture of SiC and ZrB.sub.2 is used, the mixture is composed of SiC of 10-60%, preferably 20% in weight and ZrB.sub.2 of 40-90%, preferably 80%, in weight of the total mixture. Through the high temperature sintering, there is produced a solid hard body composed of polycrystalline fine grains of SiC or Si.sub.3 N.sub.4. The ceramic grain in the resulting body has a size of equivalent diameter 0.5-5 .mu.m and 2 .mu.m in average, although its shape is not always spherical but is sometimes spiky.
An example of a current collector using the conductive ceramics described above will now be explained with reference to FIG. 10.
As shown in FIG. 10, the current collector comprises a collector shoe 1, acting as one sliding member, and a collector ring 2, acting as the other sliding member, having a surface extending in the direction of its movement. The two sliding members are made of conductive ceramics. The collector shoe 1 is pressed against the surface of the collector ring 2 with a pressure P to make sliding contact therewith.
With the current collector of the above construction, variation of contact voltage drop V was monitored and measured over period of time T when the collector was used and it was found that the contact voltage drop V varied greatly as shown in FIG. 12. The inventors of this application investigated a cause for this great variation in the contact voltage drop and found that differently shaped grains 1C and 2C of the conductive ceramics were irregularly aggregated to form finely rugged contact surfaces of the collector shoe 1 and collector ring 2, as best seen from FIG. 11 which is an enlarged view of a portion B including the sliding surfaces, and concluded that the rugged contact surfaces cause the great variation in the contact voltage drop V.